Private DNS connectivity check

Implement connectivity check to DNS-over-TLS servers, checking that the
RFC-defined port on the host is reachable and a TLS handshake can be
performed.

Bug: 112982691
Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testPrivateDnsPolicy
Change-Id: I1eb4ec201d7e096b969b7bc2bcba271f99de2d2f
6 files changed